You need the FA exe editor, to fix the bug in the game when using a router and flying multiplayer...

Due to this bug, the game is not being able to distinguish between the public, and private, IPs used when running a router, because of network address translation (NAT) in the router. This can be overcome when you run this tool and overwrite a segment of the FA.exe code with your external (public) IP number...

You will then need to run this tool again whenever your external (public) IP changes, which usually happens every 6 months or so...